Question

A pith ball of mass $9\times {10}^{-5}kg$ carries a charge of 5$\mu C$. What must be charge on another pith ball placed directly 2 cm above the pith ball such that they are held in equilibrium?

• A. $3.2\times {10}^{-11}C$
• B. $7.84\times {10}^{-12}C$
• C. $1.2\times {10}^{-13}C$
• D. $1.6\times {10}^{-19}C$

Answer 1

The correct option is B $7.84\times {10}^{-12}C$

For equilibrium

F=q6

F=mg

$\begin{array}{c}F=\frac{KqBQA}{r^2}\left(r=0.02m_{1}k=\frac{1}{4\pi \in u}\right)\\ \frac{KqBqA}{{\left(0.02\right)}^2}=9\times {10}^{-5}\times 9.8\\ \frac{9\times {10}^9\times qB\times 5\times {10}^{-6}}{4\times {10}^{-4}}=9\times {10}^{-5}\times 9.8\\ q=7.84\times {10}^{-12}C\end{array}$